Close-up View of Two Members of the Mallow Family:
Exotic Hibiscus & The Lowly Hollyhock
(Hibiscus rosa-sinensis & Althaea rosea)
by Brian Johnston (Canada)
of the mallow family (Malvaceae)
exist world-wide, but most of the approximately 1000 species are found
in South America. The species which has the greatest economic
importance is Cotton; many of the other species grow wild, or are
cultivated as garden flowers. Both the Hibiscus and Hollyhock
belong to this latter group.
The Exotic Hibiscus
The largest group of plants in the
Malvaceae family is the genus Hibiscus.
Many species possess large, colourful and frankly, spectacular
blooms. Approximately 200 different species are known - each with
a uniquely coloured or structured flower. The name “Hibiscus” is
derived from the Greek hibiskos,
and refers to the common “marsh” mallow plant. There is some
difference of opinion about whether the subject of this article, Hibiscus rosa-sinensis
is a naturally occurring species, or a collection of man-made
hybrids. The plant has been cultivated for so long that its
origins are in doubt.
As you can see below, Hibiscus
leaves are intensely green, ovate in shape and roughly toothed.
An immature bud is shown
below. Five to ten bracts
(modified leaves) are visible that form a whorl around the calyx
enclosing the flower. The calyx
is the outermost whorl of a flower, and in the bud stage, it helps
protect the petals and other structures as they develop.
When the flower begins to open, the
calyx can be seen to have five pointed “teeth”. The green colour
of both bracts and calyx indicates that chlorophyll is present, and
that food can be synthesized in these structures.
The following images show the
elongation of the petals that make up the corolla of the flower. It is the
brightly colored corolla that aids in pollination by attracting insects
once the bloom is fully open. The calyx and corolla together form
Notice the striking sculptural
quality possessed by the calyx.
The unopened corolla is also very
sculptural, due to its prominently veined structure.
Once the Hibiscus plant begins to
bloom, the reason for its great popularity is evident. The large
colourful flowers are striking and very identifiable.
Unfortunately, they last for only a single day in most species -
opening in early morning and wilting by the evening. The flowers
studied in this article have a single whorl of five petals making up
the corolla, and are referred to as the “single form”. (“Double form”
species have a second whorl of petals.) It is interesting to note
that a Hibiscus flower, when cut from the plant, will not wilt until
its natural closing time!
The unusual reproductive structures
of an Hibiscus bloom can be seen below. The pale pink tube
arising from the centre of the flower is the “staminal column” or “stamen tube”. Many fine filaments grow out from this tube,
and each filament supports a pollen encrusted anther (the male pollen generating
organ). The staminal column is actually a thin-walled tube, and
it encloses a long, slender white style that branches at the tip.
Each style branch tip holds a bright red, round stigma pad (the female pollen
accepting organ). The third image shows each stigma pad to be
covered in fine “hairs”.
With the petals removed, it is much
easier to distinguish the features mentioned above. If you look
closely at the two images that follow, you should be able to see the
pointed pink teeth at the very top of the staminal column through which
the style emerges.
A higher magnification reveals the
many filaments topped by anthers, which seem to be arranged randomly
over the column’s surface.
A still higher magnification shows
the curled shape of an anther, and the coating of bright yellow pollen
Under the microscope, the anther’s
curled shape and connection to the filament can be seen. Each
pollen grain’s rough surface, and many spikes, are just visible.
Using a shorter focal length
objective results in an image which displays a grain’s detail more
As mentioned earlier, each of the
flower’s stigma pads is covered with what looks like fine red “hair”.
Under the microscope, these
protuberances can be seen more easily.
Strangely, some of the hairs at the
junction of stigma to style are incompletely coloured. (It looks
like gaps in a red-liquid filled column.)
If the light is positioned
properly, the very three-dimensional character of the surface of each
petal is accentuated.
The front surface of the petal is
covered with prominent pink veins that radiate out from the centre.
The back surface is more
three-dimensionally veined than the front.
This is more evident at a higher
For comparison purposes, here is a
sequence showing the opening of a flower in another hibiscus
hybrid. Although very similar to the previous plant, this one has
a darker, more solid colouration.
Notice the packing of the many
anthers in the image below. In this partially opened bud, the
copious pollen grains seen in earlier images are absent.
The Lowly Hollyhock
If you compare the images that
follow with those of the Hibiscus, the similarities are very
evident. There are however, differences.
Unlike the Hibiscus, the
Hollyhock’s leaves are divided and edged with rounded lobes. They
are also not as thick and shiny as those of the Hibiscus.
A Hollyhock bud possesses the same
outer ring of bracts, and inner calyx, but they are lighter green and
more hirsute (hairy).
The calyx in an open flower lacks
the interesting sculptured shape of the Hibiscus form. The petals
show the same prominent veining.
Hollyhock has very similar
reproductive structures. Here, however, the pollen grains are so
large and tightly packed, that it is impossible to see the anthers and
their filaments hidden beneath. The “clumps” of pollen, do
however indicate their location.
It is just possible to resolve
individual round white pollen grains in the more highly magnified image
Under the microscope, the details
of a spiked spherical pollen grain can be seen.
Like the Hibiscus, the Hollyhock
has five hairy stigma pads. Here however, they are white instead
of bright red.
The two photomicrographs that
follow show the protuberances on the stigma’s surface.
The purpose of these protuberances
is illustrated below. Their form is ideal for the “catching” of
In my opinion, some flowers are
beautiful, but not particularly interesting, because they hide
structural details. The Rose and Carnation are examples.
The Hibiscus, (and Hollyhock to a lesser degree), are both beautiful and interesting, with their display
of unique reproductive structures. (The Passion-flower would be
an over-the-top example of this latter type.)
The photographs in the article were
taken with an eight megapixel Canon 20D DSLR with Canon EF 100 mm f 2.8
Macro lens. An achromatic close-up lens (Canon 250D) was used to
obtain higher magnification in some of the images. This lens screws
into the 58 mm filter threads of the EF 100 mm Macro lens. The
photomicrographs were taken with a Leitz SM-Pol microscope (using
dark-ground condenser), and the Coolpix 4500.
Microscopy UK or their contributors.
Published in the
February 2007 edition of Micscape.
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